Researches have been made on technology for hydrogen evolution through reduction of hydrogen ions using sunlight. This technology produces hydrogen gas at a cathode-side photochemical electrode containing a conductive oxide.
However, in conventional photochemical electrodes, it is not possible to produce hydrogen gas highly efficiently, unless an electrical bias is applied.    Patent Document 1: Japanese National Publication of International Patent Application No. 2012-505310    Patent Document 2: International Publication Pamphlet No. 2010/125787    Patent Document 3: Japanese Laid-Open Patent Publication No. 2012-148216